This invention relates to blends of enhanced hydrolytic stability containing linear aromatic carboxylic polyesters comprising a bisphenol wherein the carboxylic acid component can be an aromatic dicarboxylic acid or an aliphatic saturated dicarboxylic acid such as oxalic or adipic acids. More particularly the invention relates to such polymer blends which contain in admixture with said polyester a cross-linked acrylate-methacrylate copolymer and which are stabilized toward hydrolytic degradation.
Linear aromatic polyesters prepared from dicarboxylic acids, especially from aromatic dicarboxylic acids, and bisphenols are well known for their suitability for molding, extrusion, casting, and film-forming applications. For example, U.S. Pat. No. 3,216,970 to Conix, discloses linear aromatic polyesters prepared from isophthalic acid, terephthalic acid, and a bisphenolic compound. Such high molecular weight compositions are known to be useful in the preparation of various films and fibers. Further, these compositions, when molded into useful articles using conventional techniques, provide properties superior to aritcles molded from other linear polyester compositions. For instance, aromatic polyesters are known to have a variety of useful properties, such as good tensile, impact, and bending strengths, high thermal deformation and thermal decomposition temperatures, resistance to UV irradiation and good electrical properties.
Aromatic polyesters which are particularly well suited for molding applications may also be prepared by reacting an organic diacid halide with a difunctional aliphatic reactive modifier, such as a glycol, and subsequently reacting this product with a bisphenol compound. The resulting polyesters have reduced melt viscosities and melting points which permits molding at temperatures within the operable limits of conventional molding apparatus (i.e. less than about 300.degree. C.) This type of glycol-modified polyester is more fully disclosed in U.S. Pat. No. 3,471,441, to Hindersinn et al.
In order to form a successful molding resin on a commercial scale, a polymer should be capable of being molded conveniently without significant degradation in physical properties. In this respect, although the aforementioned aromatic polyesters generally display excellent physical and chemical properties, a persistent and troublesome problem has been their sensitivity to hydrolytic degradation at elevated temperatures. This sensitivity to the combined effects of heat and moisture is also exhibited in commercially available polycarbonate resins as evidenced by the desirability of reducing the water content of the resin to less than about 0.05% prior to molding. Unfortunately, however, the aromatic polyester resins often display a more pronounced tendency to rapidly degrade and embrittle than do polycarbonate resins. This is demonstrated by the loss of tensile strength which can occur when an aromatic polyester resin is molded and subsequently immersed in boiling water. This tendency may be explained, in part, by the hydrolysis of the ester linkages under these conditions. In any event, it is to be appreciated that sensitivity to moisture represents a significant problem in aromatic polyester resins that would significantly limit their commercial utility in applications such as in autoclaves or at elevated temperatures in humid atmospheres.
It is known to add polymers and copolymers of acrylic and methacrylic esters of aliphatic and cycloaliphatic alcohols to a linear saturated aromatic polyester, derived from an aliphatic or cycloaliphatic glycol, to enhance the impact resistance of the polyesters.
Thus, for example, L. Brinkman et al., U.S. Pat. No. 3,591,659 discloses mixtures containing a linear saturated aromatic polyester derived from (1) a saturated aliphatic and cycloaliphatic diol and an aromatic dicarboxylic acid (optionally containing a small quantity of aliphatic dicarboxylic acid) and (2) about 0.05% to 25% by weight (calculated on the weight of the mixture) of a polymer of acrylic, methacrylic, or ethacrylic acid esterfied with a saturated aliphatic or cycloaliphatic alcohol, said mixtures having improved impact resistance. The mixtures contain polyesters derived exclusively from aliphatic and cycloaliphatic glycols (e.g. polyesters such as polyethylene terephthalate) which are known to be distinctive from the present bisphenol-derived polyesters. The reference does not disclose polyesters which are stabilized to hydrolysis and does not disclose that the addition to a polyester of a cross-linked acrylate-methacrylate copolymer as described by this invention.
Also, U.S. Pat. No. 3,594,450 of W. Herwig et al. discloses mixtures of linear saturated aromatic polyesters of the type described in the aforementioned Brinkman et al. patent, and 0.05% to 25% by weight (based on the weight of the mixture) of copolymers of esters of acrylic, methacrylic or ethacrylic acid and a saturated aliphatic or cycloaliphatic alcohol and a conjugated aliphatic diene e.g. 1,3 butadiene. The compositions of the patent have enhanced resistance to impact. As in the case of the preceeding Brinkman et al. patent, this patent relates to linear saturated aromatic polyesters known to be distinctive in properties from the bisphenol-derived polyesters of the invention and does not disclose that the compositions of the patent are stabilized toward hydrolysis. The patent also does not disclose addition to a polyester of a cross-linked acrylate-methacrylate copolymer.
Accordingly, it is a principal object of this invention to provide aromatic polyester compositions which are substantially stabilized toward hydrolysis while retaining the superior physical and chemical properties of the polyester.